1. Field of the Invention
This invention relates to a method of migrating a melt of a metal through a solid body of semiconductor material by thermal gradient zone melting (TGZM) and, in particular, to the uniform initiation of migration by enhancing the penetration of melts into the material at the surface where fine liquid wires of an array are being migrated.
2. Description of the Prior Art
W. G. Pfann in U.S. Pat. Nos. 2,813,048 and 2,739,088 describes methods for practicing the moving of melts of metal through particular regions of a solid body of semiconductor material by thermal gradient zone melting. However, molten line and droplet instability resulted in the breakup of the migrating lines and droplets and consequently acceptable semiconductor devices were not always obtainable.
Recently, Thomas R. Anthony and Harvey E. Cline discovered that preferred planar orientations of the surfaces of the body of semiconductor material, migration axis and line orientation axes relationship were also a necessity to migrate liquid metal wires and/or droplets through the solid body (See U.S. Pat. Nos. 3,899,362 and 3,904,442, for example.) These improvements in TGZM resulted in commercialization of the process. However, as the width of the lines being migrated become smaller, the penetration of fine liquid lines of less than 2 mils in width, and preferably 1 mil in width, and small liquid droplets, less than 6 mils in diameter, from the surface of a wafer or body of semiconductor material has been difficult to achieve repeatedly on a commercial basis by a thermal gradient alone. Although a thermal gradient is strong enough to cause migration of the small liquid zones once they are formed in the bulk of semiconductor material, the thermal gradient force is not powerful enough to overcome the surface tension forces holding fine liquid zones, or wires, on the surface of a body, or wafer. Further improvements to the TGZM processing techniques included alloying the deposited metal to the surface (U.S. Pat. No. 3,897,277) and sintering of the same (U.S. Pat. No. 4,006,040). However, the problem still persists as one attempts to migrate fine wires on a commercial basis. As a result, TGZM to date has been limited to line dimensions typical of solid state power device isolation grids and has not had any commercial impact on integrated-circuit type devices which require a much finer size of doped region.
The attempt to migrate a cross-hatched pattern of fine aluminum lines (.ltoreq.0.002") in &lt;100&gt; silicon results in the breaking up of the pattern. The breaking-up of the pattern may result in lines of one direction migrating while those intersecting therewith at 90.degree. do not migrate. If both line directions do initiate, they then break away from the intersection of two intersecting lines, and the intersection migrates independently as square droplets. The end result is a discontinuous p-doped region which is not suitable for semiconductor device applications.
The feasibility of achieving uniform initiation and substantially straight uniform doped trails when thermally migrating fine lines of aluminum in silicon has been accomplished and is described in U.S. patent application, Ser. No. 942,123 entitled "Straight, Uniform Thermal Migration of Fine Lines" (Houston, Anthony and Cline) filed concurrently with this application and assigned to the same assignee. Preferably, this is accomplished by migrating a melt in &lt;100&gt; material by TGZM processing with the thermal gradient directed slightly off-axis. One of the advancing {111} faces is then at a higher average temperature than the others. Dissolution proceeds preferentially at the hotter face and provides stability to the advancing liquid.
Although these recent findings open up new applications for the TZGM process, there are further applications requiring cross-hatched patterns. One means of getting around the problem of such patterns breaking up would be to perform two sequential straight line migrations at 90.degree. with respect to each other. This, however, not only requires twice the time in the TGZM apparatus, but also requires two aluminum, or other suitable metal, evaporations, and two photolithography operations. This repetition of processing steps adds to the cost and cuts down on the yield.
Therefore, it is an object of this invention to provide a new and improved method for fabricating grid patterns in a matrix body of semiconductor material wherein each line wire of the grid is .ltoreq.0.002" in width.
Another object of this invention is to provide a method for migrating a grid pattern, wherein in each line wire is .ltoreq.0.002" in width, through a matrix body of semiconductor material by thermal gradient zone melting.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.